Recently, techniques of forming polycrystalline silicon film at relatively low temperatures are proposed. These techniques can form polycrystalline silicon film on glass substrates of relatively low heat resistance, and thin film transistors (TFT) using polycrystalline silicon film as the active semiconductor film can be formed on a glass substrate.
Thin film transistors using polycrystalline silicon film are usable as switching elements for the pixels of active matrix liquid displays (LCD).
Thin film transistors using polycrystalline silicon film as the active semiconductor films have higher carrier mobility than thin film transistors using amorphous silicon film as the active semiconductor films, and can realize high speed operation. Thus, thin film transistors using polycrystalline silicon film are usable as the switching elements for pixels, but also as the switching elements for peripheral circuits. Accordingly, the technique which can form polycrystalline silicon film at relatively low temperatures can provide a system on panel having the display and the peripheral circuits formed on one and the same substrate.
Thin film transistors using polycrystalline silicon film as the active semiconductor films are expected to be used in liquid crystal displays, but also in organic ELs (ElectroLuminescence) displays.
As a technique which can form polycrystalline silicon film on a glass substrate at relatively low temperatures, the following technique, for example, is proposed.
First, an amorphous silicon film is formed on a glass substrate.
Next, pulsated laser beams are applied to the amorphous silicon film. The laser beams are, e.g., excimer laser beams.
When pulsated laser beams are applied, the silicon melted by the laser beams grows into crystal during solidification, and polycrystalline silicon film is formed.
However, the polycrystalline silicon film formed by the above-described technique does not have sufficiently large grain diameter of the silicon crystal. Accordingly, the carrier mobility cannot be sufficiently high.
As a technique for obtaining higher carrier mobility, the following technique is proposed.
First, an amorphous silicon film is formed on a glass substrate.
Next, the glass substrate is placed on an X-Y stage.
Next, with laser beams of continuous waves being applied to the amorphous silicon film, the glass substrate is displaced by the X-Y stage for the laser beams to scan glass substrate. The laser beams are laser beams of semiconductor excitation.
When the laser beams scan, the amorphous silicon film is melted in a region where the laser beams are being applied, and in a region where the laser beams have been applied, the silicon goes on solidifying. The crystallization of the silicon goes on in the scanning direction of the laser beams, and elongated crystal grows along the scanning direction of the laser beams. Such manner of the crystal growth is called lateral growth.
The thus crystallized silicon thin film is used as an active semiconductor film with the longitudinal direction of the silicon crystals agreed with a direction of carrier transfer, whereby a thin film transistor of very high carrier mobility can be fabricated. When the carriers are transferred in the longitudinal direction of the silicon crystals, the transfer of the carriers is not blocked by the crystal grain boundaries.
However, in crystallizing all of the solidly formed amorphous silicon film by this proposed technique, the film often peels. FIG. 23 is a view illustrating peeling of the film. As illustrated in FIG. 23, film peeling 102 takes place at a part of the crystallized silicon thin film 100. The peeling 102 of the crystallized film does not take place easily at the point where the scanning of the laser beams started and more tends to take place farther away from the point where the scanning of the laser beams started. The film peeling 102 is continued as the scan of the laser beams goes on, and the film peels over a wide area. Accordingly, the silicon thin film 110 having the film peeling 102 is unusable in products. The cause for the film peeling 102 is not clear but will be due to impurities contained in the film, the surface tension of the melted silicon, etc.
As a technique which can prevent the film peeling 102 is proposed a technique that amorphous silicon film is patterned in islands in advance, and laser beams scan the amorphous silicon film patterned in the islands (see Patent References 1 and 2).
FIG. 24A illustrates an example of the arrangement of 60 μm×70 μm rectangular island patterns 104. FIG. 24B illustrates an example of the arrangement of 50 μm×200 μm island patterns 104b. The island patterns 104b have semicircular ends. The shape and the dimensions of the island patterns are not limited the above and are suitably set.
The amorphous silicon film are patterned in islands in advance, whereby the laser beams scan the island patterns 104a, 104b one by one over a relatively small distance, and the film does not easily peel. Even when the film peels in one island pattern 104a, 104b, the island patterns 104a, 104b where the film has peeled are isolated from the rest island patterns 104a, 104b, and the peeling of the film is never taken over the rest island patterns 104a, 104b. 
The amorphous silicon film is thus patterned in island in advance, whereby the yield can be improved.
However, when the amorphous silicon film patterned in islands is crystallized, good crystals do not grow at an edge part 106 of the island pattern 104. The part which is suitable for the active semiconductor film of thin film transistors is limited to a central part 108 of the island pattern 104 (see FIG. 25A).
Accordingly, as illustrated in FIG. 25B, the island pattern 104 is further patterned to use the central part 108 alone of the island pattern 104 as the active semiconductor film 110 of thin film transistors. Then, a gate insulation film (not illustrated) is formed on the active semiconductor film 110, and a gate electrode 112 is formed. Thus, a thin film transistor 114 is fabricated. Thus, only a part of the island pattern 104 can be used as the active semiconductor film 110 of the thin film transistor 114. Accordingly, the technique of patterning the amorphous silicon film in island in advance cannot form the thin film transistors 114 dense. A technique which can form a semiconductor thin film of good crystals with high yields without pattering the amorphous silicon film in island in advance has been expected.
Following references disclose the background art of the present invention.
[Patent Reference 1]
Specification of Japanese Patent Application Unexamined Publication No. 2003-86505
[Patent Reference 2]
Specification of Japanese Patent Application Unexamined Publication No. 2003-86509
[Non-Patent Reference 1]
Nobuo SASAKI, Akito HARA, Fumiyo TAKEUCHI, Katsuyuki SUGA, Michiko TAKEI, Kenichi YOSHINO, and Mitsuru CHIDA, “A New Low-Temperature Poly-Si TFT Technology Realizing Mobility above 500 cm2/Vs by Using CW Laser Lateral Crystallization (CLC),” The Transactions of the Institute of Electronics, Information and Communication Engineers C, Vol. J85-C No. 8, pp. 601-608 (2002).
[Non-Patent Reference 2]
A. Hara, F. Takeuchi, and N. Sasaki, “Selective Single-Crystalline-silicon Growth at the Pre-defined Active Regions of TFTs on a Glass by a Scanning CW Laser Irradiation,” IEEE IEDM 2000 Tech. Digest, pp. 209-212 (2000).
[Non-Patent Reference 3]
A. Hara, Y. Mishima, T. Kakehi, F. Takeuchi, M. Takei, K. Yoshino, K. Suga, M. Chida, and N. Sasaki, “High performance Poly-Si TFTs on a Glass by a Stable Scanning CW Laser Lateral Crystallization,” IEEE IEDM 2001 Tech. Digest, pp. 747-750 (2001).
[Non-Patent Reference 4]
Y. Sano, M. Takei, A. Hara, and N. Sasaki, “High-Performance Single-Crystalline-Silicon TFTs on a Non-Alkali Glass Substrate,” IEEE IEDM 2002 Tech. Digest, pp. 565-568 (2002).
[Non-Patent Reference 5]
K. Yoshino, M. Takei, M. Chida, A. Hara, and N. Sasaki, “Effect on Poli-Si Film Uniformity and TFT Performance of Overlap Irradiation by a Stable Scanning CW Laser,” Proc. 9th Int. Display Workshops '02 (Hiroshima, Dec. 4-6, 2002), pp. 343-346 (2002).
An object of the present invention is to provide a semiconductor thin film crystallization method which can form semiconductor thin film of good crystals with high yields without patterning in island in advance.